At a time when serial music was dominant in American composition, Diamond was a neo-Romantic with an ear for melody. In an interview with the Seattle Times last month, he said, "I have always thought music had to have strong melodic contours, good rhythmic variety and counterpoint, or it would make no dent on people.... Our society needs consonance; it was always a must, because of the communicative power of that kind of music."

Diamond's many works included 11 symphonies, ten string quartets, art songs, choral music, solo prices for piano and string instruments, sonatas, and more. Perhaps his most-performed piece was Rounds for String Orchestra.

Born in Rochester in 1915, Diamond studied for a year at the Eastman School of Music before leaving for New York, where he worked with Roger Sessions at the New Music School and Dalcroze Institute. In 1935, he traveled to Paris, where he studied with Nadia Boulanger.

In the 1940s, he returned to the United States and wrote the first of his important works, including his first four symphonies and first three string quartets—the third of which won the 1947 New York Music Critics' Circle Award—as well as Rounds. He moved to Italy in the 1950s and remained there until 1966. Shortly after his return to the United States, Leonard Bernstein and the New York Philharmonic premiered his Symphony No. 5 and his Piano Concerto. Diamond, Bernstein said at the time, was a "vital branch in the stream of American music."

Diamond was chairman of the composition department at the Manhattan School of Music in the late 1960s; he taught at the Juilliard School from 1973 to 1997. He received the William Schuman medal, the gold medal of the American Academy of Arts and Letters, the Edward MacDowell Award, and the National Medal of the Arts.

According to the Democrat and Chronicle, Diamond did not wish to have a funeral, but Seattle Symphony conductor Gerard Schwarz, a champion of his music, plans to organize a memorial concert.